1. Field of the Invention
This invention relates to resonant sensors of the type known as double-tuning-forks (DTFs). Such devices can be likened to a pair of tuning forks with their tines secured end-to-end. More particularly, this invention relates to such sensors which are coupled to an electrical circuit such as an oscillator producing output signal corresponding to the resonant frequency of the DTF, which frequency in turn is responsive to a condition such as applied force.
2. Description of the Prior Art
Resonant sensors of the DTF type have been known for many years. These devices commonly are made from piezoelectric materials such as crystalline quartz. The resonant frequency typically is determined by coupling the piezoelectric excitation electrodes to an oscillator which produces an oscillatory signal at the resonant frequency of the DTF. The frequency of that signal can be measured in any of various ways. There is extensive prior art describing pertinent characteristics of DTF sensors as well as the many applications that have been proposed.
Because the resonant frequency of a DTF is a function of force applied to the element, many applications involve force measurement, e.g. to measure the differential-pressure developed across an orifice plate to determine the flow rate of a fluid developed in an industrial process. For a sampling of prior art disclosures relating to DTFs, reference may be made to U.S. Pat. Nos. 3,238,789; 4,215,570; 4,299,122; 4,321,500 and 4,372,173.
DTF sensors offer important advantages when used to measure force. In particular, because DTFs have a very high mechanical Q, they are able to make force measurements with high resolution. Also, such sensors can be manufactured economically by use of photolithographic techniques of the kind which have been extensively developed in the semiconductor industry.
Such DTF sensors typically are very small in size. For example, one unit for measuring forces up to 300 grams was one-half inch long, with tines slightly less than 0.01" square and one-third inch long. With such small size, DTFs are relatively fragile, and it has been found from experience that some percentage of them can be expected to break in usage, such as due to excessive shock or vibration. Particularly sensitive to breakage are the vibratory beams or tines of the fork, because of their small cross-sectional dimensions, and due to the fact that inclusions can develop during the crystal growth processes and small pits can develop during the chemical etching processes.
Breakage of a single tine can create a severe problem because the remaining tine may continue to vibrate, but at a frequency different from that obtained when both tines were vibrating. Such continued vibration can occur in prior art DTFs because the piezoelectric excitation electrodes are activated through connection leads which branch out to provide two parallel circuits running down both tines away from the area where connections are made to the external lead wires. Thus if one tine fractures, electrodes on the remaining tine are still energized so as to maintain vibration. When one tine is broken, the stress on the intact tine doubles, and its resonant frequency increases. Accordingly, the output of the instrument will change, so as to produce an erroneous measurement, possibly double that of the true measurement. This is because a DTF with both tines active, or with just one of its tines intact, has approximately the same vibratory frequency as a function of applied stress, so that the increased stress on the intact tine produces a corresponding change in vibratory frequency.
Another undesirable mode of failure can occur when the DTF fractures in a lead-carrying region at the remote end of the crystal, i.e. at the end remote from the external wire-connection area. In this case, both tines would remain intact and can resonate at the zero stress frequency because the sensor mechanism is unable to apply stress to the resonator, thereby falsely indicating zero force input from the process.
Of particular concern is that such errors may not be evident in any way to the user of the instrument. Such erroneous measurements can of course cause serious trouble in many applications, as where the sensor is used as an integral part of an overall instrumentation system for an industrial process or the like. Thus, a solution to this problem of tine breakage is urgently needed.